1. Field of the Invention
The present invention relates to a rechargeable battery pack provided with a function enabling accurate judgement of the lifetime of a nickel-metal halide type battery (Ni-MH type battery) or other rechargeable battery cell.
2. Description of the Related Art
In recent years, extensive use has been made of rechargeable battery cells as sources of power for various types of electronic devices. Rechargeable battery cells however deteriorate in performance due to repeated charging and discharging or along with the elapse of time. Therefore, when using a rechargeable battery cell as a backup power supply to deal with service interruptions of commercial power such as seen in the uninterruptible power supplies for servers in network systems, it is important to obtain an accurate grasp of the battery performance and in turn the battery lifetime.
In the past, as a means for evaluating the performance of rechargeable battery cells (degree of cycle deterioration), for example, Japanese Unexamined Patent Publication (Kokai) No. 11-329512 discloses to measure the charge time required for reaching a full charge state when charging a rechargeable battery cell by a constant current and compare it with the charge time at the time of new product so as to determine the degree of deterioration of performance. It is difficult however to ensure uniform charging conditions of rechargeable battery cells at all times. Further, the charge time required for charging a rechargeable battery cell to the full charge state differs according to the temperature of the battery cell.
As opposed to this, Japanese Unexamined Patent Publication (Kokai) No. 2000-12104 discloses a method of finding the residual battery capacity by determining the battery life from the internal resistance, battery voltage, and battery temperature of a rechargeable battery cell measured using a constant current load. To measure battery information (internal resistance) using a constant current load, however, a constant current load constituting an extra load on the rechargeable battery cell becomes necessary and therefore the configuration becomes bulkier and more expensive. Further, at the time of measurement, the current (charge current) of the rechargeable battery cell is wastefully consumed.
An object of the present invention is to provide a rechargeable battery pack of a simple configuration enabling accurate judgement of the lifetime of the rechargeable battery cell under stable conditions without wasteful consumption of the charge capacity of the rechargeable battery cell, that is, without discharge of the rechargeable battery cell.
The present invention takes note of the facts that the closed circuit voltage Von when charging a rechargeable battery cell and the open circuit voltage Voff of a rechargeable battery cell when opening the charge current circuit differ and that the voltage difference between them is closely related to the internal resistance of the rechargeable battery cell.
Therefore, the rechargeable battery pack according to the present invention is provided with a rechargeable battery cell comprised of for example a nickel-metal halide type battery, a battery voltage detecting means for detecting a terminal voltage of the rechargeable battery cell, a full charge detecting means for detecting a full charge state when charging the rechargeable battery cell, a switch device interposed in a charge current circuit of the rechargeable battery cell and turned off to open the charge current circuit when the full charge detecting means detects a full charge state, a memory for storing a closed circuit voltage Von of the rechargeable battery cell detected by the battery voltage detecting means immediately before the charge current circuit is opened, an internal resistance detecting means for finding a value corresponding to an internal resistance of the rechargeable battery cell from an open circuit voltage Voff of the rechargeable battery cell detected by the battery voltage detecting means and the closed circuit voltage Von stored in the memory after the charge current circuit is opened, and a lifetime indicating means for predicting the lifetime of the rechargeable battery cell in accordance with a value corresponding to the internal resistance of the rechargeable battery cell and outputting information L on the lifetime.
That is, the rechargeable battery pack according to the present invention detects the full charge state of the rechargeable battery cell when charging the rechargeable battery cell and measures the closed circuit voltage Von immediately before stopping the charging and the open circuit voltage Voff after stopping the charging to ensure uniform measurement conditions (battery state) and finds the value Rin corresponding to the internal resistance of the rechargeable battery cell as
Rin=A(Vonxe2x88x92Voff)
from the closed circuit voltage Von immediately before stopping the charging and the open circuit voltage Voff after stopping the charging to accurately judge the lifetime L of the rechargeable battery cell.
The rechargeable battery pack according to the present invention may further be provided with a current detecting means for detecting a charge current with respect to the rechargeable battery cell, store in the memory a closed circuit voltage Von of the rechargeable battery cell detected by the battery voltage detecting means and a battery charge current Ion detected by the current detecting means immediately before the charge current circuit is opened, and find a value corresponding to an internal resistance of the rechargeable battery cell as
Rin=B(Vonxe2x88x92Voff)/(Ionxe2x88x92Ioff)
from an open circuit voltage Voff of the rechargeable battery cell detected by the battery voltage detecting means and a current Ioff detected by the current detecting means after the charge current circuit is opened and the closed circuit voltage Von and battery charge current Ion stored in the memory.
At this time, it is preferable for the internal resistance detecting means to use a timer circuit to measure a time elapsed from the time of detecting the full charge state of the rechargeable battery cell and find an open circuit voltage Voff of the rechargeable battery cell or its current Ioff when a predetermined time elapses from the time of detecting the full charge state. Further, it is preferable to provide a means for prohibiting processing for calculating the internal resistance o of the rechargeable battery cell by the internal resistance detecting means when a discharge current of the rechargeable battery cell is detected by the current detecting means so as to prevent erroneous judgement of the battery life.
Further, preferably the rechargeable battery pack is further provided with a battery temperature detecting means for detecting a temperature T of the rechargeable battery cell and a correcting means for correcting the value corresponding to the internal resistance of the rechargeable battery cell found by the internal resistance detecting means in accordance with the detected battery temperature and the battery lifetime is judged by anticipating changes in the battery characteristics due to the battery temperature. Further, it is possible to operate the switch device to repeatedly close and open a charge current circuit for the rechargeable battery cell and detect the states of the rechargeable battery cell immediately before the charge current circuit is opened and after the charge current circuit is opened to eliminate variations in the measurement data at the time of detection of the status and evaluate the lifetime of the rechargeable battery cell with a high accuracy.
In addition, various techniques proposed in the past can be suitably employed for the full state detecting means. Various modifications can be made within the scope of the invention.